Decontaminating water tanks
StarBright makes an excellent water tank cleaner that I have used with very good results. It has a foaming action that picks up contaminants not only in the tank but the water lines as well.
I used this product two years ago and the water, which had been really smelly, is now very good in taste and smell.
It comes in a gallon container. I had to use 1 1/2 gallons to clean three tanks holding 180 gallons of water.